1. Field of the Invention
The present invention relates to a color cathode-ray tube and a method for producing the same. More specifically, the present invention relates to a technique to improve the stability of convergence of three electron beams in a color cathode-ray tube.
2. Description of Related Art
A conventional color cathode-ray tube includes a glass bulb (envelope) having a funnel composed of a funnel portion and a neck, and a panel, and an inner space of the glass bulb is kept under vacuum. A phosphor screen is formed on an inner wall of the panel. An electron gun is housed in the neck. On an outer circumferential surface of the funnel, a deflector deflecting electron beams emitted from the electron gun is provided. Furthermore, an inner conductive film electrically connected to an anode is formed in the funnel portion and in a part of the neck.
The electron gun includes a first electrode group (beam generating portion) for taking out electron beams and controlling a beam shape, and a second electrode group (including a plurality of focusing electrodes and an anode) for finally focusing the electron beams on the phosphor screen. Generally, three electron beams arranged in a line, composed of a center electron beam and two side electron beams on both sides of the center electron beam, are emitted from the electron gun. In the color cathode-ray tube, the three electron beams are converged on the phosphor screen, and each of the three electron beams is focused on the phosphor screen. However, due to the change with time in a potential (hereinafter, which also may be referred to as a “neck potential”) of an inner wall of the neck, the convergence state of the three electron beams varies with time. This causes color displacement. More specifically, the neck potential generally has a potential distribution depending upon the position of the neck, which forms an electric field (hereinafter, which also may be referred to as a “penetration electric field”) that penetrates each gap between the electrodes of the electron gun. The electric field acting on each electron beam is a complex electric field defined by the electric field formed by each electrode and the penetration electric field depending upon the neck potential. Therefore, if the penetration electric field changes, the complex electric field also changes, which varies the path of each electron beam. In particular, the two side electron beams are likely to be influenced by the change in the penetration electric field, so that the paths thereof vary more significantly compared with that of the center electron beam. Consequently, the landing positions of the three electron beams are shifted to cause a convergence drift, leading to color displacement.
Hereinafter, the reason why the penetration electric field changes will be described. The inner conductive film having the same potential as that of the anode is formed on an inner wall of the glass bulb, so that the neck potential immediately after the application of a predetermined voltage to each electrode has a potential distribution in which a potential decreases from an end of the inner conductive film on the neck side to an end of the neck on an opposite side of the panel. However, with the passage of time, floating electrons generated in an inner space of the neck strike the inner wall of the neck, and secondary electrons larger in number than that of the struck floating electrons are released from the neck. This increases the neck potential gradually. Consequently, the complex electric field acting on each electron beam changes with time.
As a technique of reducing the convergence drift caused by the change in the penetration electric field, a configuration is known in which a conductive film is allowed to adhere to a region on the inner wall of the neck opposed in a horizontal direction to the gap between two electrodes (focusing electrodes) other than an electrode (anode) supplied with the highest voltage among the electrodes constituting the second electrode group (e.g., see JP 10(1998)-188843 A). The horizontal direction is the same as an arrangement direction of the three electron beams. Hereinafter, this configuration will be referred to as a “conventional example”.
In the above-mentioned conventional example, although the conductive film is formed in the region on the inner wall of the neck opposed in the horizontal direction to the gap between the focusing electrodes, the conductive film is not formed in a region on the inner wall of the neck opposed in the horizontal direction to the gap between the anode and the focusing electrode (hereinafter, referred to as an “anode-side focusing electrode”) closest to the anode. Therefore, the effect of reducing a convergence drift is small. This is because, in the penetration electric field to each gap between the electrodes constituting the second electrode group, the penetration electric field to the gap between the anode-side focusing electrode and the anode most contributes to a convergence drift. The reason for this is as follows. First, a region on the inner wall of the neck opposed to the anode-side focusing electrode is close to the inner conductive film, so that the region is charged to a relatively high potential. Thus, the intensity of the penetration electric field to the gap between the anode-side focusing electrode and the anode is large, and its change is large. Second, by applying a predetermined voltage to each electrode, a main lens is formed between the anode-side focusing electrode and the anode. If the electric field distribution constituting the main lens is changed by the penetration electric field, even if the change in the electric field distribution constituting a lens between the other electrodes can be suppressed, a convergence drift may occur.
As a method for reducing the influence of the penetration electric field on the gap between the anode-side focusing electrode and the anode, decreasing the gap between these electrodes can be considered. However, this method is not preferable because the withstand voltage characteristics are degraded (e.g., a spark is generated between the electrodes).